The output impedance of a transmitter device is important because the impedance may affect the ability of the transmitter to transmit efficiently, or without error, to a receiver device. For example, it is often desirable to match the output impedance of the transmitter to the input impedance of the receiver in order to maximize power transfer or minimize signal reflections. Thus, transmitters are often designed to meet a target output impedance. In practice, however, the actual output impedance rarely matches the target output impedance exactly. Failure to meet the target output impedance may be attributed to limits on the ability of a manufacturing process to accurately set the impedance of transmitter components such as resistors or transistors. Environmental conditions, such as changes in temperature, also contribute to differences in output impedance.
When one needs to cover a wide range of output impedances, the conventional solutions are unsatisfactory for various reasons including cost of implementation and accuracy. One approach is to place an array of identical resistors in parallel to form branches connected to the output. Each resistor is paired in series with a metal-oxide semiconductor field-effect transistor (MOSFET). Activating a transistor contributes a parallel impedance to the output. The parallel impedance is equal to the sum of the impedance of the transistor and the impedance of its paired resistor. The output impedance can thus be adjusted by changing the number of enabled branches. For example, to increase the output impedance, fewer branches are enabled. Using this approach, the change in output impedance as a function of the number of activated transistors is highly nonlinear. Specifically, the impedance change associated with increasing or decreasing the number of enabled branches is proportional to 1/n, where n is the number of branches required to achieve a target impedance. Therefore, the step size is small at low target impedance values, but very large at high target impedance values.
Another approach uses resistors in series with adjustable MOSFETs. The impedance contributed by the MOSFETs is adjusted by changing the number of MOSFETs that are activated. Resistors are linear elements, MOSFETs are not. To increase the linearity, the relative contribution of the resistors to the output impedance must therefore be increased in comparison to the contribution of the transistors. However, changing the relative contributions in this manner leaves little room for adjusting the output impedance using the transistors, especially when considering the wide range of the MOSFET impedance over process variations. The range of output impedances that can be achieved is therefore limited under this second approach.